1. Field of the Invention
The present invention relates to a frequency converter and more particularly, to a frequency converter for a video tape recorder (VTR), which can be implemented as an integrated circuit.
2. Description of the Prior Art
Conventionally, a VTR, particularly a home VTR, has been adapted such that a carrier chrominance signal of 3.58 MHz in a video signal is generally converted into a low-frequency signal of 629 KHz and recorded on a tape, and the reproduced low-frequency converted chrominance signal of 629 KHz is reconverted into the carrier chrominance signal of 3.58 MHz at the time of reproduction.
A frequency converter for such a conventional VTR is disclosed in, for example, Japanese Patent Publication No. 36089/1975, Japanese Publication "Semiconductor Hand book, Mololithic Bipolar Integrated Circuit Edited by Sanyo Electric Co., Ltd., 1985" issued in Mar. 20, 1985, pp. 910, and U.S. Pat. No. 4,524,380 (particularly, FIG. 23 and the description thereof) issued in June 18, 1985.
FIG. 1 is a circuit diagram showing an example of a low-frequency converter for low-frequency converting a carrier chrominance signal of 3.58 MHz into a chrominance signal of 629 KHz at the time of recording in the conventional home VTR, which is proposed in Japanese Utility Model Application No. 174,364 filed in Nov. 13, 1985 by the inventor.
Referring now to FIG. 1, description is made on a structure of a low-frequency converter for the conventional VTR.
In FIG. 1, a color video signal is inputted to a terminal 1, and a carrier chrominance signal f.sub.s of 3.58 MHz therein is extracted by a band-pass filter (BPF) 2 and applied to a first frequency converter 3 and a burst gate circuit 4. The burst gate circuit 4 extracts a burst signal in the applied carrier chrominance signal and applies the same to a phase comparator circuit 5. The phase comparator circuit 5 compares phases of a continuous wave f.sub.s of 3.58 MHz applied from a crystal voltage-controlled oscillator (VXO) 6 and the above described burst signal applied from the burst gate circuit 4, generates a DC voltage corresponding to the difference therebetween and applies the same to the VXO 6, and controls an oscillation frequency of the VXO 6 so that an oscillating output signal of the VXO 6 is in synchronization with the burst signal. As a result, an oscillating output signal f.sub.S which is in synchronization with the burst signal is applied to a second frequency converter 7. On the other hand, a horizontal synchronizing signal f.sub.H in the video signal is applied to a phase locked loop (PLL) frequency multiplier 9 from a terminal 8. Accordingly, the PLL frequency multiplier 9 outputs a signal f.sub.SL (629 KHz) having a frequency which is constant times (40 times) a frequency of the above described horizontal synchronizing signal and applies the same to the second frequency converter 7 through a phase shift circuit 10. The second frequency converter 7 generates signals of f.sub.S .+-.f.sub.SL in response to the output signal f.sub.S of the VXO 6 and the output f.sub.SL of the phase shift circuit 10 and applies the same to a band-pass filter (BPF) 11. The BPF 11 extracts only a signal component of the f.sub.S +f.sub.SL in the applied signals and applies the same to a first frequency converter 3. As a result, the first frequency converter 3 generates signals of f.sub.S .+-.(f.sub.S +f.sub.SL) in response to the output signal f.sub.S from the BPF2 and the output f.sub.S +f.sub.SL of the BPF 11 and applies the same to a low-pass filter (LPF) 12. The LPF 12 extracts only a difference component, that is, a signal component of f.sub.SL in the applied signals and outputs the same through a terminal 13. More specifically, according to the circuit shown in FIG. 1, the carrier chrominance signal f.sub.S of 3.58 MHz inputted to the terminal 1 is low-frequency converted into the chrominance signal f.sub.SL of 629 KHz and outputted from the terminal 13.
As described in the foregoing, the signal of f.sub.S +f.sub.SL (=4.21 MHz) (the output of the BPF 11) which is in synchronization with the burst signal in the carrier chrominance signal and the horizontal synchronizing signal in the video signal is produced, whereby the carrier chrominance signal is low-frequency converted, in order to form a precise low-frequency converted chrominance signal f.sub.SL corresponding to 40 f.sub.H in the input video signal. More specifically, at the time of reproduction of the VTR, a fluctuation component of the time base such as jitter is generally included in a luminance signal and the low-frequency converted chrominance signal due to expansion and contraction of a tape and rotational irregularity of a video head. The fluctuation component of the time base can be removed based on the fluctuation component included in the horizontal synchronizing signal and the low-frequency converted carrier chrominance signal in the reproduced signal. Actually, there may not be necessarily sufficient correlation between the frequencies of a carrier chrominance signal and the horizontal synchronizing signal in a video signal to be recorded. When such a video signal is recorded and reproduced, there is a possibility that displacement and fluctuation of the carrier chrominance signal is compensated for too much or too less. In addition, in a VTR of a VHS system, when the low-frequency converted chrominance signal is recorded, a phase of the above described low-frequency converted chrominance signal is shifted by 90.degree. every 1 H (H is a horizontal synchronizing period), to reduce crosstalk between adjacent tracks. More specifically, when the phase of the low-frequency converted chrominance signal is shifted by 90.degree., a frequency spectrum of the low-frequency converted chrominance signal between adjacent tracks is interleaved by 1/2.multidot.f.sub.H, so that crosstalk can be reduced. Thus, it is desirable that the above described low-frequency converted chrominance signal is precisely in synchronization with the horizontal synchronizing signal. Thus, in the structure shown in FIG. 1, the low-frequency converted chrominance signal including the same fluctuation component as that of the horizontal synchronizing signal in the recorded signal is obtained, so that the above described problem can be solved.
FIG. 2 is a circuit diagram showing an example of the frequency converter for converting the low-frequency converted chrominance signal of 629 KHz recorded on the tape into the carrier chrominance signal of 3.58 MHz at the time of reproduction in the conventional VTR.
In FIG. 2, a frequency modulated (FM) luminance signal and a low-frequency converted carrier chrominance signal reproduced from the tape are inputted to a terminal 21, and only the low-frequency converted chrominance signal of 629 KHz therein is extracted by an LPF 22 and applied to a first frequency converter 23. On the other hand, an oscillating output signal of a voltage controlled oscillator (VCO) 24 generated at a free running frequency of approximately 160 f.sub.H (f.sub.H : 15.74 KHz) is applied to a side lock preventing circuit 26 utilizing as a reference signal the reproduced horizontal synchronizing signal f.sub.H inputted from a terminal 25, where the frequencies of the oscillating output signal and the reference signal are compared. A correcting voltage is fed back to the VCO 24 from the side lock preventing circuit 26. In addition, the oscillating output signal 160 f.sub.H of the VCO 24 is frequency-divided into 1/4 by a frequency divider 27, so that a signal of f.sub.SL =40 f.sub.H is applied to a second frequency converter 29 through a phase shift circuit 28. On the other hand, an oscillation circuit 30 outputs an oscillating output signal f.sub.S of 3.58 MHz and applies the same to the second frequency converter 29 and a phase comparator circuit 35. The second frequency converter 29 generates signals of f.sub.S .+-.f.sub.SL in response to an output f.sub.SL of the phase shift circuit 28 and the output f.sub.S of the oscillation circuit 30 and applies the same to a BPF 31. The BPF 31 extracts only a signal component of f.sub.S +f.sub.SL in the applied signals and applies the same to the first frequency converter 23. As a result, the first frequency converter 23 generates signals of (f.sub.S +f.sub.SL).+-.f.sub.SL in response to the output signal f.sub.SL of the LPF 22 and an output signal f.sub.S +f.sub.SL of the BPF 31 and applies the same to a BPF 32. The BPF 32 extracts only a difference component, that is, a signal component of f.sub.S in the applied signals, outputs the component through a terminal 33 and applies the same to a burst gate circuit 34. The burst gate circuit 34 extracts a burst signal in the applied carrier chrominance signal f.sub.S and applies the same to a phase comparator circuit 35. The phase comparator circuit 35 compares phases of the signal f.sub.S of 3.58 MHz applied from the oscillation circuit 30 and the above described burst signal applied from the burst gate circuit 34, generates a voltage corresponding to a difference phase therebetween and applies the same to the VCO 24, and controls the oscillation frequency of the VCO 24 so that the burst signal is in synchronization with the phase of the signal from the oscillation circuit 30.
As described in the foregoing, according to the circuit shown in FIG. 2, the low-frequency converted chrominance signal of 629 KHz inputted to the terminal 21 is converted into the carrier chrominance signal of 3.58 MHz and outputted from the terminal 33.
However, in the circuit shown in FIG. 1, the frequencies of two output signals of the second frequency converter 7 are relatively approximate values, that is, f.sub.S +f.sub.SL =4.21 MHz and f.sub.S -f.sub.SL =2.95 MHz, so that the BPF 11 having high Q is required for the determination. In addition, in a second circuit, the frequencies of the two output signals of the second frequency converter 29 are also approximate values, that is, 4.21 MHz and 2.95 MHz, so that the BPF 31 having high Q is required for the determination. Since it is generally difficult to contain such a band pass filter having high Q in an IC, the number of parts externally provided is increased and the number of pins is increased, so that the frequency converter can not be implemented as an IC.